The present invention concerns a process and a plant for handling thermoformed objects particularly for a single-station thermoforming machine with form and cut mould.
In the thermoforming field of hollow packaging objects, and in particular for objects of relatively low height and medium or large dimensions, such as lids or containers, for example round and rectangular plates, tubs and the like, it is becoming of crucial importance for the manufacturer to be able to supply the market with thermoformed products, which have been correctly stacked, in perfect axial arrangement and easily de-stackable or removable from the stack. This necessity is increasingly important as the growth of the automatic product packaging sector continues. This means that the stacks of thermoformed products are usually destined to feed an automatic packaging machine and, if the stacks have defects in their axial arrangement or the products in their stacks do not have a constant and uniform index between them, or one or more thermoformed products is stuck in the next or previous container in the stack, impeding or making the separation difficult, the packaging machine stops and needs an operator to intervene, with consequent down time, increase of waste and then packaging costs.
To resolve this problem and in a particular that one of the hollow thermoformed products"" reciprocal sticking, it has already been suggested to provide undercut recesses (b) in hollow thermoformed products (a) on the rim or on the wall or lateral walls (c) or on the base (f) of the thermoformed products (see FIG. 1 of attached drawings), such recesses being obtained by providing corresponding fixed or mobile undercutting lugs or projections in the thermoforming mould.
However, if the recesses in the undercut recesses (b) on the side wall or walls are too prominent, the extraction of the thermoformed product from the thermoformed mould becomes difficult; if, instead, they are not very prominent, they are unable to prevent the stacked thermoformed products sticking to each other.
On the other hand, it is difficult, if not prohibitive, to constantly and precisely check a certain number of parameters, namely: thickness, density, fluidity grade etc., of a sheet or belt of thermoformable material in the mould of a thermoforming machine, and obtain therefore uniform and dimensionally perfect undercut recesses in the thermoformed containers.
The recesses obtained with mobile projections in the mould are very expensive to produce and to keep working efficiently. Experience has shown that the mobile projections in the mould frequently stop because of the inevitable dirtying due to the plastic material dust which always accrues in the form and cut mould of the thermoforming machine.
In any case, then, it is known that in stacks of hollow thermoformed products (a) with vibrations and shaking during transport or warehouse handling, the undercutting recesses (b), if not perfect, cause the thermoformed containers or lids making up the single stacks to stick.
For all these reasons, instead of obtaining stacks axially aligned and with thermoformed products (a) kept distant a uniform distance (d) as shown in FIG. 1, uneven stacks are formed with thermoformed products (a) arranged roughly and therefore not uniformly distanced from one to the other (FIG. 2), in as much as the thermoformed products (a ) are inevitably subject to sticking to each other, which makes it difficult, if not impossible, to de-stack or separate them before use. Clearly, a stack of thermoformed products (a) not perfectly aligned in an axial direction and with products (a) that are difficult to de-stack, cannot be used in automatic distribution machines or in automatic filling lines for thermoformed containers.
The same problems occur with the undercutting projections (b) when they are provided on the base (f) of thermoformed products, since the projections often stick to each other with the result that the product stacks are defective, as shown in FIG. 3 of the drawings.
The main object of the present invention is to provide a process and a handling and stacking plant for a plurality of thermoformed objects that can eliminate the difficulties mentioned above with the technical solutions available in the state of art.
According to a first aspect of the present invention there is provided a process for handling and stacking a plurality of thermoformed objects, which is characterized in that it comprises the following sequential phases:
obtaining thermoformed objects or mouldings of thermoformed objects each formed with at least three stacking protrusions or spacers having the same space arrangement in each thermoformed object of the same moulding, at least one stacking protrusion or spacer of a same thermoformed object being located in a non specular symmetric way with respect to at least a centre line of the respective thermoformed object and at a distance from the same centre line different from that of the other protrusions or spacers,
arranging one thermoformed object or a moulding of said thermoformed objects in at least one support template to keep them in order according to the spatial arrangement that they had during the thermoforming phase,
turning through a predetermined angle around a vertical axis every other thermoformed object or moulding of thermoformed objects before or during their transfer to a stacking station, and
stacking thermoformed objects or mouldings of thermoformed objects with alternate objects or mouldings turned through said predetermined angle to obtain stacks of objects, where the stacking protrusions of an object are offset with respect to those of the next object in each stack.
According to another aspect of this invention there is provided a plant for handling and stacking thermoformed objects having at least three projections acting as stacking spacers, at least one of which is arranged at non specular symmetry with respect to at least a centre line of the respective object, said plant including, in sequence, a receiving station for a thermoformed object or a moulding of thermoformed objects, at least one stacking or working station for said thermoformed objects, a stacking station for said thermoformed objects, means for transferring the thermoformed objects from the receiving station to the stacking station through each working or handling station, wherein at least one of the said handling stations includes handling means arranged to rotate through a predetermined angle about a vertical axis every other moulding of thermoformed objects before or during their transfer to said stacking station, thereby obtaining stacks of thermoformed objects lids or containers, where the stacking spacers of an object container or lid are angularly offset with respect to those of the next object.
Advantageously, the transfer devices include a step rotating conveyor.